This invention relates to improvements in support brackets for use in collapsible steering column assemblies for vehicles to permit collapse during a crash.
Steering column assemblies for automotive vehicles and the like are increasingly required to be adjustable for steering wheel height known as rake and, in many cases, steering wheel distance known as reach. This requires the column shroud, within which the steering column shaft attached to the steering wheel is rotatably located, to be fixed to the vehicle by a clamp mechanism which can be locked and unlocked to either prevent or permit adjustment of the column shroud position respectively.
One common arrangement uses a telescopic column shroud which comprises an inner member and an outer member—typically two elongate metal tubes—which slide one inside the other to permit reach adjustment. Fixing rails are welded to one of the tubes and are secured to a support bracket by a releasable clamp mechanism. The support bracket is in turn secured to a relatively immobile part of the vehicle chassis. When the clamp mechanism is clamped the fixing rail and support bracket are fixed relative to one another. When unclamped they can move relative to one another to permit the required reach adjustment. Rake adjustment can be achieved by providing a support bracket of which the lower part known as the U bracket is shaped like an inverted U and includes vertically extending slots in its side arms through which the clamp mechanism passes. The clamp mechanism can be moved vertically within these slots, taking the column with it to adjust rake.
In the event of a crash it is desirable for steering column assembly to collapse in a controlled manner. This helps to reduce the forces of a driver colliding with the steering wheel or its airbag. In one known arrangement this controlled collapse is enabled by frangibly connecting the U bracket to the vehicle using one or more shaped blocks called capsules. In a crash the forces on the U bracket are sufficiently high to cause it to break away from the capsules thus permitting the U bracket to move relative to the vehicle. Often some form of energy absorbing device fitted between the U bracket and the vehicle body controls the movement of the support bracket. For example, a deformable strap may be fixed to the U-Bracket and may slide over a fixed anvil, causing progressive distortion of the strap in order to absorb energy.
FIG. 2 shows a typical prior art arrangement of a support bracket which permits this type of movement in a crash. It comprises a two part support bracket, the first bracket part being in the form of the said U bracket fixed to the clamp mechanism and the second part being in the form of an upper mounting bracket fixed to the vehicle (although this other part could be a fixed part of the vehicle body) with the two parts connected via capsules. FIG. 5 shows a typical prior art arrangement of a capsule. The capsule comprises a main body part that is cast or machined in the form of two larger plates between which is sandwiched a small central portion. A profile cut out from the first bracket part is typically designed to be a close fit to a corresponding male profile which is formed in the central portion of the capsule. After assembly of the capsule to the wing of the first bracket part, a frangible material such as a plastic is usually injected through holes in the first bracket part and the capsule which are co-axial. There may be more than one group of such coaxial holes. Once set, the plastic acts as a fusible dowel which is designed to shear when the said prescribed level of force is applied. Typically, the plate thickness of the wing of the first bracket part is slightly less than the gap between the upper and lower plate portions of the capsule so that a proportion of the injected plastic spreads into the said gap and improves the stiffness of the connection between the capsule and the first bracket part.
The fusible capsule defines an anchor point for a fastener which allows the first bracket part to be rigidly fixed to the second bracket part. This is usually by way of an additional large hole in the second bracket part and a coaxial hole in the capsule which receives a fastening bolt. In a crash the first bracket part breaks away from the capsule. When assembled the first and second parts of the bracket are separated by the thickness of the upper plate of the capsule or capsules. The applicants have appreciated that this sandwich construction leads to an undesirable thickness for the support bracket for some applications where space is limited.